The present invention relates to an air conditioner having a refrigerant circuit, which includes a condenser, an expansion valve, an evaporator and a variable displacement compressor.
A typical refrigerant circuit in a vehicle air-conditioning system includes a condenser, an expansion valve, an evaporator and a compressor. The compressor is driven by a vehicle engine. Thus, the compressor is preferably a variable displacement type, which draws, compresses and discharges refrigerant gas, the flow rate of which corresponds to the cooling load regardless of the engine speed. A typical commercialized variable displaced compressor is a variable displacement swash plate type compressor. A swash plate type compressor generally includes a swash plate located in a crank chamber and a displacement control valve. The compressor uses the control valve to change the pressure in the crank chamber to vary the displacement. The pressure at the outlet of the evaporator, which is referred to as pressure Psxe2x80x2 in this specification, represents the cooling load. The control valve adjusts the crank pressure to change the inclination angle of the swash plate to control the compressor displacement. Accordingly, the pressure at the evaporator outlet seeks a target suction pressure, or a pressure that represents a desired compartment temperature. The swash plate type compressor includes a swash plate supporting mechanism, which couples the swash plate to pistons and permits the swash plate to tilt. Lubricant oil is retained in the compressor and is converted to mist by gas circulating in the compressor. The oil mist is then supplied to locations where relative motion and contact occur between parts of the supporting mechanism. Some of the oil is also discharged from the compressor with gas through the crank chamber. The discharged oil is however returned to the compressor via the refrigerant circuit, which maintains a sufficient amount of oil in the crank chamber.
The evaporator receives refrigerant mist from the expansion valve and performs heat exchange between the mist and the air of the passenger compartment to cool the air. The refrigerant mist is heated and vaporized, however, insufficient vaporization has various disadvantages such as a reduction in the cooling efficiency. Thus, the refrigerant is preferably vaporized and superheated at the outlet of the evaporator. One of the basic functions of an expansion valve is to feedback control the flow rate of refrigerant supplied from the condenser to the evaporator such that the refrigerant at the evaporator outlet has a superheat (SH) of ten degrees centigrade maximum.
A typical prior art variable displacement swash plate type compressor changes its displacement based on the pressure Psxe2x80x2 at the evaporator outlet. This type of compressor must be combined with a cross-charge type expansion valve and not with a normal-charge type expansion valve. A first broken line, which is formed by short dashes, in the graph of FIG. 3 represents the saturation vapor pressure characteristics of refrigerant. A cross-charge type expansion valve modifies the characteristics of the refrigerant to those represented by a second broken line, which is formed by long and short dashes. The first broken line and the second broken line intersect at a point (t, p). If the cross-charge type expansion valve is used, the characteristics are shifted to higher pressures from the original saturation vapor pressure characteristics line (the first broken line), and no superheat is present, in the range of temperatures and pressure that are lower than the point (t, p). In this state, refrigerant is liquefied at the outlet of the expansion valve. A normal-charge type expansion valve modifies the characteristics of the refrigerant to those shown by the solid line. The normal-charge type expansion valve superheats the refrigerant in the entire pressure and temperature range. Thus, if the normal-charge type expansion valve is used, refrigerant is always gasified at the outlet of the expansion valve.
Conventionally, the swash plate type compressor, which varies its displacement, is used together with a cross-charge type expansion valve. This is because the flow rate of lubricant that is returned to the compressor via the refrigerant circuit must always be equal to or greater than a certain level. Specifically, when the temperature T and the pressure Psxe2x80x2 of the refrigerant at the outlet of an evaporator are high due to a high cooling load, the expansion valve increases the flow rate of refrigerant in the refrigerant circuit, and the displacement control valve increases the compressor displacement. If the flow rate of refrigerant in the circuit is greater than a certain level, lubricant is reliably returned to the compressor and, in this state, the type of the expansion valve does not matter. However, when the temperature T and the pressure Psxe2x80x2 of the refrigerant at the outlet of the evaporator are low due to a small cooling load, the expansion valve decreases the flow rate of the refrigerant in the circuit, and the displacement control valve decreases the compressor displacement. If a normal-charge type expansion valve is used, the flow rate of refrigerant from the outlet of the evaporator to the compressor is limited, and the refrigerant carries a small amount of lubricant oil. Thus, if the low cooling load state is extended, the amount of oil discharged from the compressor will surpass the amount of oil returned to the compressor, which will deplete the oil in the compressor. This is why a cross-charge type expansion valve is typically used for a refrigerant circuit. When the cooling load is low, or the temperature at the outlet of the evaporator is lower than that of the intersection point (t, p), liquefied refrigerant, which contains lubricant oil, is returned to the compressor from the evaporator.
However, returning liquefied refrigerant to the compressor from the evaporator degrades the cooling efficiency. When liquefied refrigerant is returned to the compressor, less of the energy used to drive the compressor is used for cooling. In other words, more of the energy that drives the compressor is used for just circulating liquefied refrigerant in the refrigerant circuit, not for the heat exchange. If a cross-charge type expansion valve is used, the super heat SH increases as the temperature and the pressure at the outlet of the evaporator are increased. Therefore, it is difficult to maintain an appropriate degree of superheating. Further, cross-charge type expansion valves are generally more costly than normal-charge type expansion valves. Thus, it has been considered desirable to avoid using a cross-charge type expansion valve in a refrigerant circuit whenever possible.
Accordingly, it is an objective of the present invention to provide an air conditioner, which includes a variable displacement compressor, that maintains a high cooling efficiency and returns a certain amount of oil to the compressor even if the flow rate of refrigerant is decreased due to a low cooling load. Another objective is to overcome disadvantages that accompany a cross-charge type expansion valve.
To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, an air conditioner is provided. The air conditioner includes a refrigerant circuit, two pressure monitoring points and a displacement control device. The refrigerant circuit includes a condenser, a normal-charge type expansion valve, an evaporator and a variable displacement compressor. The pressure monitoring points located in the refrigerant circuit. The displacement control device determines a target difference of the pressure difference between the pressure monitoring points and feedback controls the displacement of the compressor such that the pressure difference between the pressure monitoring points seeks the target difference.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.